. Published data demonstrate that lowering of the serum tyrosine concentration reduces the number of metastases in various animal models and in human cancer patients. The reduction is achieved by placing the patient/animal on a reduced-tyrosine diet. Because these diets are not palatable and administration of a purified tyrosine-destroying enzyme achieved equally positive results, the long-term goal of this work is to clone, express, and evaluate therapeutic enzymes that catabolize tyrosine.